Alkylene oxides, for example, ethylene oxide, propylene oxide and butylene oxide, have been subjected to liquid-phase hydration to produce the corresponding alkylene glycols. Commercially, in the production of ethylene glycol from ethylene oxide large molar excesses of water are used (See, Kirk-Othmer, Encyclopedia of Chemical Technology Volume 11, Third Edition, Page 939, (1980)). It has been reported that the presence of large quantities of water in the reaction system are necessary if the yield to the desired monoalkylene glycol is to be great enough to be commercially viable and minimize the production of by-products such as diglycols and triglycols. Accordingly, the commercial practice has generally involved the hydration of an alkylene oxide at a temperature of about 100.degree. C. to about 200.degree. C. in the presence of a large molar excess of water, for example, in excess of 15 moles of water per mole of alkylene oxide, when the corresponding monoalkylene glycol is to be produced. Unfortunately, the use of such large excesses of water presents significant energy and equipment requirements for its removal.
since the selectivity of the hydration process to monoglycol, e.g., ethylene glycol, propylene glycol or butylene glycol, is dependent on the by-products formed, it would be desirable to provide a process that would increase the selectivity of the hydration process to monoglycol products. In addition, any process which would favorably decrease the relative amount of water employed to alkylene oxide hydrated while not increasing, or preferably decreasing, the by-products formed would be advantageous. Thus, the energy and equipment requirements would necessarily be less for separation and purification processes relating to the removal and recovery of the monoglycol from water and by-products.
Several processes have been suggested for the hydration of an alkylene oxide in the presence of a specific catalyst such that the ratio of water to alkylene oxide may be lowered and such that the selectivity to monoglycol product may be maintained or enhanced.
Numerous catalysts have been suggested for use in the hydration of alkylene oxides, including the use of acid catalysts such as: alkyl sulfonic acid ion exchange resins (U.S. Pat. No. 4,165,440); carboxylic acids and halogen acids (U.S. Pat. No. 4,112,054); strong acid cation exchange resins (U.S. Pat. No. 4,107,221); aliphatic monocarboxylic and/or polycarboxylic acids (U.S. Pat. No. 3,933,923); cationic exchange resins (U.S. Pat. No. 3,062,889); acidic zeolites (U.S. Pat. No. 3,028,434); sulfur dioxide (U.S. Pat. No. 2,807,651); Ca.sub.3 (PO.sub.4).sub.2 (U.S. Pat. No. 2,770,656); high-melting polyvalent metal fluorides (U.S. Pat. No. 2,547,766); trihalogen acetic acid (U.S. Pat. No. 2,472,417); and copper-promoted aluminum phosphate (U.S. Pat. No. 4,014,945).
In addition to the acid catalysts, numerous catalysts have been suggested for the hydration of alkylene oxides in the presence of carbon dioxide. These include alkali metal halides, such as chlorides, bromides and iodides, quaternary ammonium halides such as tetramethyl ammonium iodide and tetramethyl ammonium bromide (British Patent No. 1,177,877); organic tertiary amines such as triethylamine and pyridine (German published patent application No. 2,615,595, Oct. 14, 1976, and U.S. Pat. No. 4,307,256, issued Dec. 22, 1981); quaternary phosphonium salts (U.S. Pat. No. 4,160,116, issued July 3, 1979); and chlorine or iodine-type anion exchange resins (Japanese Kokai No. 57/139,026, published Aug. 27, 1982); and partially amine-neutralized sulfonic acid catalyst, e.g., partially amine-neutralized sulfonic acid resin (U.S. Pat. No. 4,393,254, issued July 12, 1983).
Although a review of the results reported in the patent literature would suggest that the above-described catalysts have provided commercially acceptable results, that is, a high selectivity to the monoglycol product and a decrease in the requirement for large molar excess of water, these catalysts have not been commercially employed for several reasons. For example, alkali metal halides tend to corrode the reaction system at the temperatures employed for the hydration of alkylene oxides. The relatively low solubility of alkali metal halides and quaternary ammonium halides in alkylene glycol restricts their use as hydration catalysts since they are likely to precipitate within the reaction system during the course of the hydration reaction and can result in problems associated with cleaning the reaction system. In addition, some catalysts, such as tertiary amines, have certain chemical and physical properties which prevent their ready use as hydration catalysts. For example, tertiary amines have a strong pungent odor which is not desirable in manufacturing and can detract from the quality of the end product.
U.S. Pat. No. 4,277,632, issued July 7, 1981, discloses a process for the production of alkylene glycols by the hydrolysis of alkylene oxides in the presence of a catalyst of at least one member selected from the group consisting of molybdenum and tungsten. The patent discloses that the catalyst may be metallic molybdenum or metallic tungsten, or inorganic or organic compounds thereof, such as oxides, acids, halides, phosphorous compounds, polyacids, alkali metal and alkaline earth metal, ammonium salts and heavy metal salts of acids and polyacids, and organic acid salts. An objective of the disclosed process is stated to be the hydrolysis of alkylene oxides wherein water is present in about one to five times the stoichiometric value without forming the appreciable amounts of by-products, such as the polyglycols. The reaction may be carried out in the presence of carbon dioxide. The patentees state that the process can be effectively carried out in the presence of from 0.00001 to 1, preferably from 0.0001 to 1, mole of carbon dioxide per mole of alkylene oxide. In the examples, where carbon dioxide was employed (either as carbon dioxide or a bicarbonate salt), the amount ranged from 0.001 to 0.08 mole of carbon dioxide per mole of ethylene oxide. When the reaction is carried out in the presence of nitrogen, air, etc. the patentees state that the pH of the reaction mixture should be adjusted to a value in the range of 5 to 10. Japanese Kokai No. JA 54/128,507, published Oct. 5, 1979, discloses a process for the production of alkylene glycols from alkylene oxides and water using metallic tungsten and/or tungsten compounds.
Japanese Kokai No. JA 56/073,036, published June 17, 1981, discloses a process for the hydrolysis of alkylene oxide under a carbon dioxide atmosphere in the presence of a catalyst consisting of a compound containing at least one element selected from a group comprising aluminum, silicon, germanium, tin, lead, iron, cobalt and nickel.
Japanese Kokai No. JA 56/073,035, published June 17, 1981, discloses a process for the hydrolysis of alkylene oxide under a carbon dioxide atmosphere in the presence of a catalyst consisting of a compound containing at least one element selected from the group of titanium, zirconium, vanadium, niobium, tantalum and chromium. The amount of carbon dioxide to be employed in the disclosed process is within the range of 0.00001 to 1, preferably, 0.0001 to 1, mole of carbon dioxide per mole of alkylene oxide. The compounds employed as the catalysts include the oxides, sulfides, acids, halides, phosphorous compounds, polyacids, alkali metal salts of acids and polyacids, ammonium salts of acids and polyacids, and heavy metal salts of acids. Although the examples show the use of various metal catalysts, the disclosure does not disclose any detail as to the nature of the hydration process and the selection of the catalysts employed therein. In example 2, the process is carried out using a potassium vanadate as the hydration catalyst for the production of ethylene glycol from ethylene oxide and water under a carbon dioxide pressure. No identification of the vanadate used was made. The carbon dioxide was provided in the amount of 0.01 mole per mole of ethylene oxide. The conversion of ethylene oxide to products is reported to be 100 percent but the selectivity to monoethylene glycol is only 50 percent. The combined selectivity to diethylene glycol and triethylene glycol is also 50 percent. Thus, example 2 shows that the use of potassium vanadate was only slightly better than the obtained 36.1 percent selectivity reported for the conversion of ethylene oxide to ethylene glycol wherein no catalyst was employed (see comparative example 1 of JA 56/073,035).
Japanese Kokai No. JA 56/92228, published July 25, 1981, is directed to processes for producing highly pure alkylene glycols. The disclosure is directed to a distillation procedure for recovery of a molybdenum and/or tungsten-containing catalyst from an alkylene oxide hydrolysis process in the presence of carbon dioxide. The application states that the catalyst is at least one compound selected from the group consisting of compounds of molybdenum and tungsten which compound may be in combination with at least one additive selected from the group consisting of compounds of alkali metals, compounds of alkaline earth metals, quaternary ammonium salts and quaternary phosphonium salts. The preferred catalysts are stated to be molybdic acid, sodium molybdate, potassium molybdate, tungstic acid, sodium tungstate and potassium tungstate. Potassium iodide is the only additive employed in the examples.
J. H. Robson and G. E. Keller, II, in copending U.S. patent application Ser. No. 530,235, filed Sept. 8, 1983, herein incorporated by reference, disclose the use of water-soluble vanadate salts at a pH of between 5 and 12 to enhance the selectivity of the hydration of alkylene oxides to monoalkylene glycols. One preferred vanadate salt comprises metavanadate, and it is reported that carbon dioxide decreases the selectivity to the monoalkylene glycol product when using this salt. Thus, when substantially all of the vanadate anion is believed to be metavanadate anion, carbon dioxide is disclosed to be desirably less than about 0.10, preferably less than 0.05, mole per mole of alkylene oxide.